1. Field of the Invention
The present invention relates to an electrode current collector that may be advantageously used in, e.g., lithium secondary battery, and a method for producing the same, an electrode for battery and a method for producing the same, and a secondary battery. More particularly, the present invention relates to the improvement of discharge capacity and cycle characteristics.
2. Description of Related Art
In recent years, mobile devices are being improved in performance and increased in functions, and, in accordance with this tendency, demands are made on secondary batteries used as a power source of the mobile devices, in which the batteries should be reduced in size, weight, and thickness, and particularly should be increased in capacity.
As a secondary battery which can meet the demands, there is a lithium-ion secondary battery. The lithium-ion secondary battery has battery properties which sharply change depending on the electrode active material used and the like. In a typical lithium-ion secondary battery being practically used, lithium cobalt oxide is used as a cathode (positive electrode) active material and graphite is used as an anode (negative electrode) active material, and the lithium-ion secondary battery having such constituents has a battery capacity reaching the theoretical capacity, and hence it is difficult to drastically increase the battery capacity by the further improvement.
For achieving a lithium-ion secondary battery having a capacity drastically increased, studies are made on the use of silicon or tin which is capable of forming an alloy together with lithium during the charging as an anode active material. However, when silicon or tin is used as an anode active material, expansion or shrinkage caused during the charging or discharging is large, and therefore the expansion or shrinkage caused during the charging or discharging causes the active material to be finely divided or to be removed from the current collector, leading to an issue of lowering of the cycle characteristics.
On the other hand, in recent years, an anode electrode including an anode active material layer made of silicon or the like stacked on an anode current collector has been proposed (see, for example, Japanese Patent Application Publication No. Hei 08-50922, Japanese Patent No. 2948205, and Japanese Patent Application Publication No. Hei 11-135115). This anode electrode is considered to unify the anode active material layer and the anode current collector, preventing the active material from being finely divided due to the expansion or shrinkage caused during the charging or discharging. Further, there can be obtained an advantage in that the electron conduction of the anode electrode is improved.
In patent document 1 (International Patent Publication No. WO01/029918, page 9), there is a description showing that, from the viewpoint of improving the adhesion between the anode active material layer and the current collector, the current collector for anode electrode on which an anode active material layer made of silicon or the like is stacked is preferably made of a metal capable of forming an alloy together with the anode active material layer, and, when silicon and germanium layers are stacked on the current collector, the current collector for anode electrode is especially preferably made of copper. In addition, there is a description showing that an electrolytic copper foil having a large surface roughness Ra is preferred as a copper foil. The electrolytic copper foil is a copper foil having both surfaces roughened, which is obtained by immersing a copper foil, such as a rolled copper foil, in an electrolytic bath to permit copper fine particles to be deposited on both surfaces of the copper foil by an electrolytic process.